Long term evolution (“LTE”) of the Third Generation Partnership Project (“3GPP”), also referred to as 3GPP LTE, refers to research and development involving the 3GPP Release 8 and beyond, which is the name generally used to describe an ongoing effort across the industry aimed at identifying technologies and capabilities that can improve systems such as the universal mobile telecommunication system (“UMTS”). The goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards. The 3GPP LTE project is not itself a standard-generating effort, but will result in new recommendations for standards for the UMTS.
The evolved UMTS terrestrial radio access network (“E-UTRAN”) in 3GPP includes base stations providing user plane (including packet data convergence protocol/radio link control/medium access control/physical (“PDCP/RLC/MAC/PHY”) sublayers) and control plane (including radio resource control (“RRC”) sublayer) protocol terminations towards wireless communication devices such as cellular telephones. A wireless communication device or terminal is generally known as user equipment (“UE”) or a mobile station (“MS”). A base station is an entity of a communication network often referred to as a Node B or an NB. Particularly in the E-UTRAN, an “evolved” base station is referred to as an eNodeB or an eNB. For details about the overall architecture of the E-UTRAN, see 3GPP Technical Specification (“TS”) 36.300, v8.5.0 (2008-05), which is incorporated herein by reference. The terms base station, NB, eNB, and cell refer generally to equipment providing the wireless-network interface in a cellular telephone system, and will be used interchangeably herein, and include cellular telephone systems other than those designed under 3GPP standards.
Orthogonal frequency division multiplex(ing) (“OFDM”) is a multi-carrier data transmission technique that is advantageously used in radio frequency based transmitter-receiver systems such as 3GPP E-UTRAN/LTE/3.9G, IEEE 802.16d/e (Worldwide Interoperability for Microwave Access (“WiMAX”)), IEEE 802.11a/WiFi, fixed wireless access (“FWA”), HiperLAN2, digital audio broadcast, (“DAB”), digital video broadcast (“DVB”), and others including wired digital subscriber lines (“DSLs”). The OFDM systems typically divide available frequency spectrum into a plurality of carriers. Each of the plurality of carriers has a narrow bandwidth and is modulated with a low-rate data stream. The carriers are closely spaced and orthogonal separation of the carriers controls inter-carrier interference (“ICI”).
When generating an OFDM signal, each carrier is assigned a data stream that is converted to samples from a constellation of admissible sample values based on a modulation scheme such as quadrature amplitude modulation (“QAM,” and its higher-order variants 16QAM, 64QAM, etc), quadrature phase shift key (“QPSK”), and the like. Once phases and amplitudes are determined for the particular samples, they are converted to time-domain signals for transmission. A sequence of samples, such as a 128-sample sequence, is collectively assembled into a “symbol.” Typically, OFDM systems use an inverse discrete Fourier transform (“iDFT”) such as an inverse fast Fourier transform (“iFFT”) to perform conversion of the symbols to a sequence of time-domain sample amplitudes that are employed to form a time domain transmitted waveform. The iFFT is an efficient process to map data onto orthogonal subcarriers. The time domain waveform is then up-converted to the radio frequency (“RF”) of the appropriate carrier and transmitted.
Delay in processing the signal in the transmitter is a concern in the design of communication systems for several reasons. First, in the case wherein the user equipment transmits the signal, the symbols should be received at the base station within a certain window of time, which the base station assures by sending a particular timing advance to the user equipment in its cell. Second, multi-path delay interference in the transmitted symbols causes inter-symbol interference (“ISI”) between a reflected radio signal and the direct radio signal. A cyclic prefix (“CP”) is a sequence of samples inserted before a symbol to separate a sequence of symbols and avoid inter-symbol interference. The time interval a cyclic prefix occupies is referred to as a guard interval (“GI”). The cyclic prefix is generally just a repetition of the ending samples of the symbol that are added at the beginning of the symbol. Inserting the cyclic prefix should be performed in a manner so that the transmitted signal is sent with the proper timing advance. To achieve a reasonable throughput, the OFDM symbol duration may be at least five times the guard interval and to avoid inter-carrier interference, the OFDM symbol is cyclically expanded in the guard interval. A cyclic prefix makes distortion effects of the channel multiplicative, thereby allowing easy compensation of channel effects at the receiver.
The cyclic prefix insertion is conventionally performed as follows. An iFFT has an associated “length” corresponding to a number of coefficients for the iFFT transform. The cyclic prefix is generated by placing the last few iFFT output coefficients (time-domain signal amplitudes) at the beginning of the symbol to form the cyclic prefix. The size of the cyclic prefix varies in different applications. For example, in LTE with a normal cyclic prefix, for an iFFT with a length “N” of 2048 coefficients, the iFFT output corresponding to the last 144 coefficients is transmitted first as the cyclic prefix, and then the output corresponding to the 2048 coefficients of the entire iFFT is output in normal order, for a total of 2048+144=2192 coefficients. For the addition of a cyclic prefix in conventional systems, the iFFT output requires relatively large buffers. If the output of the iFFT is in bit-reversed time order, then two buffers of size N are required. If the output of the iFFT is in normal time order, then a single buffer of the length N of the iFFT can be used. In either case, buffers add expense to the communication system.
Conventional multicarrier OFDM systems have a particular drawback of a high peak-to-average ratio (“PAR”) in the resulting transmitted waveform, thereby increasing requirements on the power amplifiers of the transmitter such as amplifier linearity and amplifier peak power rating. The need to transmit a waveform with a high peak-to-average ratio is usually not acceptable for handheld devices such as user equipment, and thus single carrier-frequency division multiple access (“SC-FDMA”) systems are often used for the link from the user equipment to the base station. The SC-FDMA systems retain many of the beneficial characteristics of the OFDM systems.
As these communication systems become more complex, the length of each of the symbols increases. In 3GPP LTE, the symbol length is 2048 samples when the cell bandwidth is 20 megahertz (“MHz”). A process for appending a cyclic prefix to a symbol to form an extended (i.e., cyclic prefix-expanded) symbol includes calculating the whole symbol in time and buffering the same during the calculation and processing of the symbol. After the symbol has been calculated and processed, the ending samples of the symbol are taken from the buffer, which are then transmitted first as the cyclic prefix, followed by transmitting the whole symbol.
Appending the cyclic prefix in this way requires the whole symbol to be calculated before its transmission can start. This delay reduces signal-processing margin for other communication operations. As an example for an LTE communication system, the roundtrip delay is four milliseconds (“ms”), which is the time between a base station transmitting a (sub)frame, user equipment receiving and processing the (sub)frame, the user equipment transmitting a response and the base station receiving the response.
When appending the cyclic prefix in this way, the whole symbol also should be buffered before a transmission can start. This means a buffer of the size of at least one symbol. In an optimal arrangement, a buffer corresponding to the cyclic prefix length would be sufficient. In a worst case, at least one quarter of the symbol length would be required, and depending on the particular implementation, a buffer sufficient to store two symbols would be necessary. Thus, reducing buffering requirements and processing latency associated with insertion of a cyclic prefix in high-frequency digital communication systems would provide an advantageous level of operation.
In view of the growing deployment and sensitivity to delays of communication systems such as cellular communication systems, further improvements are necessary for reducing buffering requirements and processing latency associated with insertion of a cyclic prefix. Therefore, what is needed in the art is a system and method that avoid the associated deficiencies of conventional communication systems in accordance with the insertion of a cyclic prefix and the like.